1. Field of the Invention
This invention relates to optical fiber connectors, and, in addition, to optical fiber connector bodies, optical fiber holders, and combinations thereof. Accordingly, it is a general object of this invention to provide new and improved devices of such character.
2. Description of the Prior Art
With some exceptions, including those listed below, previous fiber optic connectors essentially have been devices for physically bringing the ends of the two fibers together so closely and accurately as to minimize the disturbance of the optical waveguide geometry of the fibers as light passes from one to the other. Disadvantageously, except for those situations involving very large diameter fibers, extreme mechanical tolerances have been required. The competing requirements of close tolerances on one hand, and permitting a large number of connect-disconnect cycles with little degradation on the other hand, are exceptionally difficult to achieve in the same device. In particular, connectors intended for small core communications-grade fibers with less than 1 dB loss have been possible only by using very expensive individually machined and aligned parts. They usually cannot be reliably installed in the field. In addition, the small fiber ends must be kept clean and free from scratches, or the optical throughput efficiency decreases rapidly; and a small particle caught between two connectors has often resulted in crushed or fractured fiber ends.
Known exceptions use lenses of various types to transfer the optical power between the ends of the two fibers physically separated from one another. All but one of these use a symmetrical imaging optical system with half of the optics in each of the connectors such that the light is more or less collimated into an expanded beam of parallel rays. The present invention falls in this category. Such an optical connector, if all of the parts of each connector are accurately aligned relative to one another and to the fiber within the connector, presents the advantage of loose tolerance of lateral offset and axial separation between the connectors, but at the cost of more stringent angular alignment tolerance between the connectors compared with the case of physically abutted fibers. The major differences among the various designs are the techniques for assuring the internal tolerances and alignment of the connector parts, the methods of attaching and aligning the fiber to the connector, and the means (if any) for assuring precise angular alignment at the interface of the two connectors. Known optical connector designs using lenses are:
a. Discrete conventional lenses with the fiber positioned at the focal point of the lens, and air between the fiber and the lens, as suggested by M. A. Bedgood, J. Leach, and M. Matthews, "Demountable Connectors for Optical Fiber Systems", Electr. Commun. (U.K.), Vol. 51, pp. 85-90, 1976. The alignment of all parts, and of the connectors to one another at the interface, are maintained by accurate machining and individual alignment procedures. Disadvantageously, a discrete lens must be carefully aligned in a precision connector housing, with its optical axis oriented accurately along the axis of the housing. Then, a fiber must be manipulated to be precisely along the same axis, with its end at the focal point of the lens. Also, the fiber and lens have three glass-to-air interfaces in each connector, which must be anti-reflection coated to achieve minimum losses.
b. Glass sphere with index of refraction very close to 2.00, the case in which collimated light from one side focuses onto a small spot on the opposite side, as suggested by A. Nicia, "Practical Low-Loss Lens Connector for Optical Fibers", Electronics Lett., Vol. 14, pp. 511-512, August, 1978. The problem of placing the fiber at the correct distance from the lens then becomes trivial, since the distance is zero and the fiber is simply abutted to the sphere surface. Index matching material may be used between the fiber and sphere lens to partially reduce reflection losses. Other alignment tolerances must be dealt with by accurate machining of parts and individual alignment procedures. Disadvantageously, this is the only critical alignment eliminated (and one of the least critical) of those listed above.
c. Rod lens used as in paragraph b above instead of the glass sphere. Numerous glass and plastic versions of this have been reported. (West Germany Patent No. 2,746,497 may fit this category; an English translation thereof has not been obtained, nor reviewed.) The rod lens is a cylindrical rod with a lens surface at one end and with the other end being a flat surface at the focal plane of the lens surface. The fiber is abutted directly to the rod lens back surface. The index of refraction of the lens material can be much lower than that of the sphere and can be much more easily index matched to the fiber index, almost completely eliminating reflection losses at these two faces. The cylindrical geometry allows a better alignment of the lenses of two connectors if both are inserted into a common close-fitting cylindrial tube. Disadvantageously, it still has all of the alignment problems of the sphere lens.
d. A quarter-pitch gradient-index rod lens, used like the uniform glass rod lens in paragraph c above. This has all of the above features, but in addition has no curved surface at the connector interface. A precise radial variation of the index of refraction of the rod material can produce much the same collimating/focusing action as the uniform rod with a curved end surface. Thus, the angular alignment of two lenses can be assured by abutting their two flat surfaces together, assuming that the lenses have end surfaces which are accurately made to be perpendicular to the lens optical axes. Furthermore, index-matching material may be used between these flat surfaces as well as between the lens and the fiber to virtually eliminate all reflection losses.
There are several disadvantages of this approach. A fluid interface between the lenses is difficult to maintain in the field, and tends to attract environmental contaminants which can scratch the surfaces or reduce transmittance; it is almost impossible to clean. Without a fluid interface, contact between the two non-index matched surfaces may produce interference effects which can cause additional reflection losses and modal noise. Furthermore, known prior art manufacturing tolerances for the index profile result in significant losses when the fiber is abutted to the rod end. And, as with the above designs, no means is provided for aligning the fiber laterally or angularly to the optical axis of the rod lens, so external precision alignment of the lens and fiber to the housing are still required.
e. Rod lens with integral fiber-alignment means. The lenses detailed in paragraphs b, c, and d, above, provide for trivial axial alignment of the fiber at the correct distance from the lens (namely, abutted directly against the lens itself). Various designs have additionally incorporated into the shape of the lens an integral cylindrical recess with substantially the same diameter as that of the fiber to be used, thus providing a measure of lateral and angular alignment of the fiber with respect to the lens optical axis. U.S. Pat. No. 4,183,618 to Rush et al., issued Jan. 15, 1980, suggests such design. West Germany Patent No. 27 22 367 to Combined Optical Ind., describes the addition of a conical entrance to the cylindrical recess to receive excess optical liquid which is squeezed out during insertion of a fiber. Both patents describe means separate from the integral lens for holding the fiber in place: Rush et al. describes a connector with longitudinally extending tensioned wires for holding the external fiber in alignment with a short length of optical fiber from the lens aperture, while West Germany Patent No. 27 22 367 describes a neoprene cushion or bolster which is squeezed to hold the fiber. Both indicate that an optical coupling material can be used in the cylindrical fiber recess. U.S. Pat. No. 4,147,402 to Chown, issued Apr. 3, 1979, describes another version, using an intense laser beam to form the fiber recess.
Disadvantageously, the cylindrical recess suffers from the same problem as butt-type connectors which use a cylindrical ferrule to align the fibers to the connector axis. Manufacturing tolerances on the outside diameter of optical fibers are sufficiently significant to require an oversized hole diameter to assure that the fiber in hand will fit into it. Though selected fibers of a maximum external diameter may assure a tight fit, as suggested by Rush et al., fibers on the small end of the tolerance range usually end up offset from the optical axis since the cylindrical recess does not center the smaller fiber. In commercial connectors which utilize a rigid cylindrical hole to align the fiber, losses due only to this effect have been observed to be often greater than 1 dB, in excess of the losses due to other tolerances. Furthermore, as with other designs above, the connector housing must provide precision means for aligning the optical axes of the lenses within very tight angular tolerances.
f. Rod lens as in paragraph e above, with an integral surface for alignment of the lenses within the connector housings, as in French Patent No. 2334969 to Cosneau, which describes an annular shoulder around the edge of the lens and recessed back from the convex lens surface. This shoulder mates with an annular surface in a separate connector housing, and another annular surface of the housing mates with a matching one on the other connector's housing. When all three pairs of matable surfaces are properly mated, the two lenses in the two connectors are automatically aligned to the extent of the sum of the mechanical errors in the lens and housing parts. Cosneau also optionally describes an integral fiber-aligning recess as in paragraph e, above, with an additional integral cavity to accept the fiber coating or sleeve, which has been stripped back only to the length of the smaller fiber recess.
Disadvantageously, the connector housing must still bear the burden of providing extremely accurate, generally individually machined or adjusted, mating reference surfaces to utilize the recessed reference surfaces of the lenses. All six of the critical mating surfaces in a connector pair must be made so accurately that the sum of their errors adds up to less than the angular tolerance needed for low-loss connection.
g. Multi-lens junction between two fiber-holding connectors as described in U.S. Pat. No. 4,119,362 by Holzman, issued Oct. 10, 1978. Optical alignment is achieved with a single molded lens part in a central junction. This part has two concave lens surfaces molded aligned to one another, one on each end. This is not an expanded-beam connector (i.e., one where the light exiting from an optical fiber is expanded into a beam many times the diameter of the fiber); the diameter of the lenses, and of the light distribution between them, are approximately the diameter of the fiber. Conical guides extend from the lens surfaces to aid in directing the fibers to the lens. They also provide precise axial centering of the fiber end for various fiber end diameters within the tolerance range of fiber manufacturing.
The lens-tipped conical indents are filled with an optical fluid or other material which has an index of refraction higher than that of the plastic lens, thus producing two additional fluid lenses bounded by the curved plastic surface and the flat fiber end. The resulting three-element complex lens produces what appears to be approximately an optical Fourier transform of the source fiber light distribution on the receiving fiber end. This transform yields potentially advantageous mode-mixing, and yields an acceptable coupling efficiency if the fiber has a parabolic graded index profile.
Disadvantageously, when the connectors are separated, the bare fiber is exposed and coated with sticky optical fluid. In practice, this fluid quickly becomes contaminated with dust and other materials from the environment and optical transmittance is reduced significantly. Also, because the fiber is pressed into the conical recess each time the connectors are joined, the multiple impacts with the sharp edge of the fiber eventually damage the plastic surface. Both problems result in the requirement that the entire central plastic lens unit be replaced quite frequently to assure continued low loss. Also, the fiber ends are exposed to damage when the connectors are separated, so retractable fiber protective means are required in the connector end housings to minimize breakage. Finally, although the conical guide surface centers fibers of different diameters, the amount of displacement of the fiber end from the lens varies with different diameter fibers. Since the lenses are so small, compared to an expanded beam connector in accordance with the invention set forth in this specification, this tolerance is much more critical.
Another version described in the Holzman patent includes another fluid "field lens" in the center in a configuration which provides an expanded beam. This is made in two parts, with one concave surface of the field lens molded on the interface surface of each half. The parts are, in theory, separable, but the field lens must be filled with high-index fluid when mated. Disadvantageously, there is no means provided for filling the field lens cavity with bubble-free optical fluid each time the connectors are to be joined, and care is required to keep the fluid from leaking out from the interface over extended periods of service.
h. Finally, the patented literature includes numerous low-precision fiber or fiber bundle interface devices, such as:
U.S. Pat. No. 3,649,098 to Suverison, issued Mar. 14, 1972, relates to a lens structure of a fiber optic assembly which is adapted to be detachably connected to an apertured panel, for example in automobile vehicle applications as an indicating means. It indicates when a remote light bulb is burned out via a fiber optic bundle.
U.S. Pat. No. 3,734,594 to Trambarulo, issued May 22, 1973, describes an optical fiber splicer having a deformable angular core disposed between a pair of metallic pressure plates. The fibers to be spliced are inserted into opposite ends of the core and a longitudinal force applied to the plates causes the core to deform radially, thereby securing the fibers.
U.S. Pat. No. 3,948,582 to Martin, issued Apr. 6, 1976, discloses an optical fiber connector with separately formed bodies of substantially elongated form. Each body has an axial bore in which the optical fibers can be fitted. The end of one body defines a socket adapted to mate with a plug-shaped end of the second body.
U.S. Pat. No. 4,056,305 to McCartney et al, issued Nov. 1, 1977, describes a connector having a deformable elastomeric alignment element having a through bore. Two sets of three equal diameter cylindrical rods are mounted in opposite ends of the bore so as to define a space therebetween for receiving an optical fiber. The rods have an interference fit in the central portion of the bore so that compression of the rods results in laterally aligning the fibers.
Some of these, generally, utilize large-diameter fibers or bundles for short communications links or simply as light guides to remote indicator panels or illumination devices. They are not relevant to the extremely tight-tolerance requirements of modern small-diameter, low-loss communications systems applications to which the present invention is addressed.